This invention relates to gas turbine machinery, and relates more particularly to self-contained starting systems carried with the machinery providing starting capabilities without utilization of external or ground support starting equipment.
Larger gas turbine machines such as gas turbine engines utilized in powering aircraft normally include a smaller gas turbine machine such as an auxiliary power unit and/or jet fuel starter which operate to assist in starting the larger engine, as well as providing power to drive auxiliary units of the aircraft. It has heretofore been found most commercially feasible and economical to provide external ground support equipment to accomplish starting of the main engine or engines. While certain types of self-contained starting systems have been developed and utilized in for highly sophisticated machinery such as military aircraft, these approaches generally have been too complicated and expensive for commercial exploitation. The overall expense, complexity, inefficiency, bulkiness and weight of such prior art self-contained start systems have severely limited their general utility. Most such self-contained starting systems have been of the hydraulic type, wherein hydraulic fluid is stored for use in starting the machinery; then during operation of the machine the hydraulic reservoir is replenished. Several drawbacks are associated with such hydraulic systems; primarily the weight and leakage restrictions of hydraulic systems have limited their utility. The hydraulic systems also encounter operational problems at low temperatures due to increased viscosity of the fluid.
While certain limited attempts have been made to provide a pneumatic, self-contained starting system, such arrangements have not met with much success primarily because of their operational efficiency. More particularly, most prior art pneumatic self-contained starting systems direct a supply of pressurized gas directly onto the compressor of the gas turbine machine for initiation of rotation thereof during start-up. Then during normal operation of the machine, the reservoir of pressurized gas is replenished. Because of the relative inefficiency of the compressor during its initial start-up, acceleration operation, it has been found that such prior art systems require an inordinately and unfeasibly large reservoir of pressurized gas to effect start-up even if the compressor of the turbine is not subject to any load. Of course to initiate acceleration of the gas turbine machine, normally a fairly substantial load is placed upon the compressor during start-up acceleration.
Other prior art types of self-contained starting systems are of the electrical type wherein stored-in electrical energy is discharged for start-up, and then stored during machine operation. Such electrical systems suffer from loss of battery power at low ambient temperatures, and generally are not sufficiently efficient systems for developing the necessary torque during start up. Particularly in aircraft installations, the substantial weight imposed upon the aircraft by a hydraulic or electrical self-contained starting system have greatly limited the applicability of each.
As a result of the general difficulties encountered in developing self-contained starting systems, most gas turbine machinery now rely either upon external equipment or throw-away, cartridge-type starters.